The issue with Firefox x64 and H.264 codecs is being investigated and as soon as a fix is available, it will be posted here.

I uninstalled Flash, reboot and re-tested the previous video per your comment. Chrome (64), Firefox (32), Opera and IE were able to play it Sandboxed.
Can you please provide a link to the exact videos that would not play in Chrome?

Also, make sure to try a new Sandbox, or delete the contents of the current one, prior to re-try playing the videos.

Regards,
Barb.-

Chrome (64) has been okay for me, perhaps just a hiccup while testing. However, Firefox (64) is most assuredly not working on many videos (all using h264) but you already know about that. Any updates on a fix for that? It is really problematic.